An extra-dimensional monster created by unknown parties. Zom claims to have been created to destroy, but the methods of his creation are unknown. Ages ago, he was defeated under unknown circumstances, which may have involved a union of several other extremely powerful beings, including Dormammu (the ruler of the Dark Dimension) and Eternity (the cosmic being representing the entire temporal axis of the Earth dimension). To protect the universe from his might, Zom's hands were bound with Links of Living Bondage by Dormammu. Zom was forced to wear a Crown of Blindness, and his essence was imprisoned by Eternity within a Legendary Amphora in a "world beyond all worlds" in a "time beyond all time."
When Doctor Strange was confronted by Umar, years ago, her power threatened to overwhelm him. The Ancient One directed him to a distant dimension to retrieve Zom, as he was one opponent even she could not defeat. But while the ploy was successful in deflecting and banishing Umar, Zom now became the threat; in the end, the demon was dispatched by the Living Tribunal himself.
Following Zom's banishment, Strange magically stitched the Amphora back together and sealed it with a spell. Clea later apparently calls on Zom for power in a spell to free Gaea from Dormammu's cursed prison at the center of the Earth along with Oshtur, but it is possible that she merely mentioned him in the spell. A small fragment of Zom remained imprisoned inside the jar, which was kept safe by Doctor Strange. On one occasion, A struggle in Doc's sanctum shattered the Amphora. A remnant of Zom's evil essence, the Zomling, was released. The only one who saw the Zomling was Wong's then-fiancee, Imei Chang, but the Zomling magically stopped her from mentioning it. The Zomling scurried around the mansion, absorbing power from various sources. After growing in power, it discovered the Faeries who inhabit the sanctum, and managed to capture the Faery Queen.
Strange discovered the shattered Amphora and sent his associates on a mission to locate the Zomling. Rintrah found it and attacked it before it could consume the queen, but Rintrah's magical attack only made it all the more powerful. However, Strange then arrived, and knowing that the Zomling fed on magical energy, and it was composed mostly of energy, but also of the dust it had consumed in the process; Strange used a vacuum cleaner to suck up and entrap the Zomling. He then magically repaired the Amphora and reversed the suction on the vacuum to spit it back into its prison. Strange resealed the Amphora, and then put it a little more out of the way, where it was unlikely to be shattered again.
World War Hulk
When faced with the unstoppable rage of the Hulk, Doctor Strange resorted to invoking Zom's essence into himself by drinking the contents of the amphora. In the following issue it was referred that seven S.H.I.E.L.D.-employed mystics stated that he did indeed channel Zom's powers, and he severely battered and damaged the Hulk, but began losing control of the demon's rage. He restrained Zom long enough to be knocked unconscious. The Zomling left Strange and began to seek a means to grow in power. According to writer Greg Pak, "In Strange's defense, he didn't release Zom in the flesh the way he did back in the day -- instead Strange channeled Zom's power through his own body. That's probably one of the reasons the Earth remains on its axis.
After Strange's defeat, the infernal entity resumed its mission to destroy the Earth dimension, and started by inhabiting Iron Man's discarded "hulkbuster" armour, to activate the latter's anti-matter doomsday device. Wong sought to recapture it into the urn, and was assisted by Hercules, Namora, the Angel, and Amadeus Cho. While it still proved too powerful to defeat, Amadeus managed to trick it into possessing his body, so the Angel could knock him out, and it was successfully resealed. Zom was referred to as severely depleted at the time.
Doctor Strange was shot and ostensibly killed by the demon-empowered Hood, who was leading an all-out assault by about fifty super-villains on the doctor's house. Strange, however, rose again and manifested what appeared to be the power of Zom to dispatch all his enemies (and friends) in one furious conflagration. After this, he left the Avengers and possibly the mortal plane to "relearn everything he lost", explaining he could not control the dark forces he unleashed, forces he had used through the unholy bond he had secretly and desperately maintained with Zom to enforce his weakening magical abilities.
Zom is one of the most formidable mystic entities in existence, referred to far exceed even Umar in terms of sheer mystical power, and thus her brother Dormammu as well, along with nearly any Sorcerer Supreme, including that of Doctor Strange and the Ancient One together. Zom's power even surpasses any member of the Council of Godheads and at least rivals any deity or demon on Earth. His full might equal to certain cosmic entities, it once took the Living Tribunal, near-supreme being of the entire Multiverse, to actually stop him, although Zom greatly feared the Tribunal, and the power difference is therefore not marginal. However, the Tribunal's power is technically infinite, and thus the combined forces of all the realities existent in Marvel's might would not be marginal compared with the Tribunal. He uses his powers primarily to destroy everything around him. Apparently his long lock of hair acted as a mystic shield which masked him from the detection of more powerful mystic entities. The removal of this forelock awakened the "sense of evil which had slumbered for ages in the bosom of the mystics of mankind." Even a portion of the power of Zom has proven easily capable of harming the Hulk, overwhelming dozens of Earth superhumans simultaneously, and resurrecting fallen sorcerers.
Though Zom, in terms of raw power, is indeed virtually unstoppable, his might is also such that he naturally attracts the attention of such beings as Eternity and the Living Tribunal, who would even destroy him forever to maintain the magical balance of all reality.